Linear ion traps are described in U.S. Pat. No. 5,420,425 to Bier et al. A particularly preferred embodiment, which is in fact applied in a successful commercial mass spectrometer, consists in assembling four hyperbolically shaped rods to create a very precise linear quadrupole system, making slots in two opposing rods, and mass-selectively ejecting the gas-cooled ions through the slots by radial resonant excitation. If the arrangement is perfectly symmetrical, the ions then emerge, during what is called a mass scan, uniformly (although in offset ion pulses, on account of the resonantly excited vibrations of the ion cloud) through the two slots in the opposing pole rods throughout the individual ion mass signals, and are measured by two flat detectors placed in front of the two slots. An ion trap of this type is shown schematically in FIG. 1, although only one of the detectors is visible.
In order to record a mass spectrum, a mass scan is required in which the operating parameters of the ion trap are changed in such a way that ions are ejected mass-selectively and mass-sequentially out of the ion trap and into the detectors where they are measured. “Mass” refers here, as is always the case in mass spectrometry, to the mass-to-charge ratio, m/z. The specialist knows several types of such mass scans, including, in particular, ejection by storage instability at the edge of the Mathieu stability diagram, and ejection of the ions by radial, resonant excitation by a dipolar RF excitation voltage. In the latter case, the resonant ejection can be supported by nonlinear resonances; this then permits particularly fast scan methods with high mass resolution, as described in U.S. Pat. No. 6,831,275. Ejection by nonlinear resonances also offers the advantage that the ions can be ejected on one side only, so that only one detector is required.
An advantage of linear ion traps over so-called three-dimensional ion traps, which consist of a ring electrode and two end cap electrodes, is that they are easier to fill and have a high capacity for ions. A disadvantage of this arrangement is the extraordinarily high precision necessary to give a constant form and intensity to the RF electrical field at every cross-section along the axis. The precision of the RF field is affected by disturbing effects at both ends of the pole rod system, disturbances at the ends of the slots, and, in particular, by the mechanical precision required for the shape and spacing of the pole rods.
Pole rods are usually used with an internal spacing of eight millimeters, that is to say an “inside radius” of four millimeters. If, at any point along the axis, this radius deviates from its specified value by as little as two micrometers, then ions with a mass of 2001 Daltons (or 1999 Daltons) are ejected instead of the desired 2000 Daltons. If ions with a mass of 1000 Daltons are to be ejected, then ions with a mass of 1000.5 Daltons (or 999.5 Daltons) are ejected at the location of the inaccuracy. This means that a mass spectrometer of this type does not offer usable resolution if it has such dimensional inaccuracies. The usable mass range is also limited, as a resolution of a single mass unit is no longer available above 2000 Daltons. In fact the mechanical precision required for the pole rods of a usable mass spectrometer is much less than a micrometer.
The requirement for a mechanical precision of well below one micrometer is, however, almost impossible to meet. Commercial mass spectrometers of this type are restricted to a mass range of 2000 Daltons, with a maximum resolution at the upper end of the mass range of about R equal to 4000, whereas commercial three-dimensional ion traps consisting of turned parts offer a mass range of 3000 Daltons along with a mass resolution of more than R equal to 10,000 at the upper end of the mass range. This difference is crucial for many applications, such as modern protein analysis.